Curved antenna with variably spaced slots



March 2, 1965 w. G. HEINARD ETAL 3,172,113

CURYED ANTENNA WITH VARIABLY SPACED SLOTS Filed June 6, 1962 2 Sheets-Sheet l 1 INVENTORS Y W/l/LDE/V 6 Ham/e0 Mil/AM A! PEPPER BY j lfio ziwz, aJQww March 2, 1965 w. G. HEINARD ETAL 3,172,113

CURVED ANTENNA WITH VARIABLY SPACED SLOTS 2 Sheets-Sheet 2 Filed June 6, 1962 United States Patent 3,172,113 CURVE!) ANTENNA WITH VARIABLY SPACED SLOTS Whilden G. Heinard and William H. Pepper, Bethesda,

Md, assignors to the United States of America as represented by the Secretary of the Army Filed June 6, 1362, Ser. No. 200,589 9 Claims. (Cl. 343-771) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment to us of any royalty thereon.

This invention relates to directional electromagnetic wave antennas, and more particularly to antennas which may be used on a moving vehicle without adverse influence on the vehicles aerodynamic characteristics.

High speed aircraft and missiles have placed severe demands on electromagnetic radiating antennas. Along with space and weight considerations, these antennas must not only have directive radiation patterns but also be able to be incorporated within the existing air frame Without producing an adverse efiiect on its flight characteristics. Slotted waveguide antennas are commonly used in such situations. These antennas are straight waveguide sections, having radiating slots in a broad face of the guide, and may be easily accommodated along a longitudinal axis of a cylindrical body, such as a missile. These antennas may be designed to have fan shaped radiation patterns in which the main lobe of the antenna lies substantially on a conical surface, with axis of the cone coinciding with the longitudinal axis of the waveguide. The planar radiation pattern is achieved by properly spacing the apertures in the waveguide and controlling their excitation. The degree to which the radiation can be limited to a single plane, or in other words the narrowness of the plane, is primarily determined by the physical length of the antenna.

Straight slotted waveguide antennas are satisfactory when it is desired to have a directive radiation pattern in a plane which intersects the longitudinal axis of a generally cylindrical air frame, such as a missile. In this case an antenna of sufiicient length can be accommodated along the outer surface of the missile parallel to its longitudinal axis. Such straight waveguide antennas are not suitable, however, where it is desired to have the fan shaped radiation pattern of the antenna contained in a plane containing, or parallel to, the longitudinal axis of a cylindnical air frame. In sucha situation there is not enough space in the transverse direction of the missile to accommodate a straight waveguide antenna with the necessary physical length to achieve sufiicient directivity, without protruding and thereby affecting the missiles aerodynamic characteristics.

A first object of this invention is to provide a novel microwave antenna with non uniform spacing between radiating slots.

A further object of this invention is to provide a slotted wave guide antenna which can be supported by a curved airframe without affecting its aerodynamic characteristics, where the antenna radiation pattern is in a plane which contains or is pmallel to the axis of curvature.

A still further object of this invention is to provide a slotted waveguide antenna which has a sharply defined null in a generally planar radiation pattern.

An additional object of this invention is to provide a slotted waveguide antenna which can alternate between a full planar radiation pattern and a planar radiation pattern having a sharply defined null therein.

These and other objects of the invention are achieved 3,172,113 Patented Mar. 2, 1965 by providing a curved slotted waveguide antenna, with the radiating slots in the broad face of the wave guide on the side away from the center of curvature.

The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawing, in which:

FIG. 1 illustrates an antenna in accordance with this invention in combination with a generally cylindrical air frame;

FIGS. 2a, 2b, 2c, are views of the air frame antenna, and radiation pattern of FIG. 1 in the X2, Y2, and XY planes, respectively.

FIG. 3 is a detailed view of the antenna of FIG. 1.

FIG. 4 is a modification of FIG. 3 wherein one arm of the antenna is fed out of phase with respect to the other arm.

The main lobe of a fan shaped radiation pattern of a straight slotted waveguide antenna, with staggered slots in the broad face of the antenna guide, is, as is known in the .art, at an angle 0 with respect to the longitudinal axis of the guide determined by along an arc of a circle are for staggered slots, that is, where the slots are on alternate sides of the longitudinal center line of the broad face of the guide, and

where the slots are all on one side of the longitudinal center line. As in (1), 6 is the angle of the plane containing the radiation with respect a plane tangent to the circumference of the guide, is the free space wave length, and A is the guide wavelength, but n is an odd integer. R is the radius of curvature of the Waveguide and (p is the angle from the feed point to the kth slot.

Referring to FIG. 1, there is shown a missile 11 with a curved waveguide antenna 13 having a generally fan shaped radiation pattern 12 in an XZ plane, that is, a plane containing the longitudinal axis of the missile 11. As can be seen in FIG. 1, by curving the antenna 13 a long antenna can be obtained that conforms to the short transverse dimension of the missile.

FIG. 3 is a detailed view of the antenna 13 shown in FIG. 1. The antenna 13 comprises a curved section of waveguide 15 having. a radius of curvature R, a center of curvature at 14, and a longitudinal center line It). The Waveguide 15 has radiating slots 16 spaced at nonuniform predetermined distances in its broad face on the side away from the center of curvature 14. The curved waveguide 15, as shown,'is center fed by a waveguide 17, through a T connection 18. The angle 0, that is, the distance from the feed point 18 to each radiating slot 16, is determined from Equation 2 or 3 above depending upon whether the slots are spaced on both sides or only one side of the longitudinal axis 10. The angle 0 is the angle between the plane of the fan shaped radiation pattern represented by 12 and a plane, represented by 19, tangent to the curved waveguide section 15. As in the case of the straight slotted waveguide antenna of the prior art, the angle may be varied between wide limits, to suit a particular application.

Referring to FIG. 2, the radiation pattern for the antenna 13 is shown for the conditions when 0 is equal to 90 and both halves of the waveguide 15 are fed in phase so that the radiation from each slot reinforces along the center line. FIG. 2a shows the radiation pattern of antenna 13 in the XZ plane of FIG. 1. PEG. 2b shows the radiation pattern of antenna 13 in the YZ plane, and FIG. shows the radiation pattern of the antenna 13 in the XY plane.

Referring to FIG. 4, the radiation pattern of the antenna 13 is shown in the YZ plane (like FIG. 2b) when the radiating slots on one side of the feed point are fed 180 out of phase with respect to the corresponding slots on the other side. In this case a sharp null is produced on the beam axis as shown by the radiation pattern 21. This sharp null can be produced either by shifting each radiating slot on one side of the T connection 18, 180 electrical degrees relative to the corresponding slot in the other arm or by incorporating a phase shifter, such as ferrite phase shifter 24, in one arm of the antenna 13. Phase shifter 24 may comprise a small section of ferrite surrounded by a controllable magnetic field. As is well known in the art, the amount of phase shift introduced by the phase shifter 24 is dependent upon the strength of the magnetic field. A suitable control signal may be derived from radio pulse transmitter 26, causing phase shifter 24 to introduce 0 or 180 phase shift in one arm of the antenna 13 on alternate pulse transmissions. A scheme of this type can be used to achieve a still greater narrowing of the eliectiveness of the radiation pattern in the YZ plane of FIG. 1, by delaying only the echo signals received from the full pattern, for example, by an interval equal to the pulse repetition rate, and subtracting undelayed echo signals from the pattern with the sharp null therefrom, whereby echo signals will substantially cancel, except echo signals from the narrow null region.

While this invention has been more particularly described in connection with a center fed waveguide, it will be apparent to those skilled in the art that the antenna may be fed if desired, from the end, and still obtain the pattern shown in FIG. 2.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made through construction and arrangement within the scope of this invention as defined by the appended claims.

We claim as our invention:

1. A slotted waveguide antenna comprising: a curved section of waveguide, said curved section of waveguide having four walls, a longitudinal center line, and a center of curvature; a plurality of radiating slots spaced longitudinally along one wall of said curved section of waveguide, said one wall being the wall facing away from the center of curvature; means to couple microwave energy to be radiated into said curved section of waveguide; the spacing between said slots varying, the variation in spacing being a function of the distance from said coupling means, whereby said antenna radiates said energy sub stantially in a single plane, said plane being orthogonal to a plane which is determined by said longitudinal axis and said center of curvature.

2. A slotted waveguide antenna as in claim 1 wherein said curved section of waveguide is an arc of a circle.

3. A slotted waveguide antenna as in claim 1 wherein where;

0 is the angle between the plane of radiation and a plane tangent to the curved section of waveguide; )t is the wavelength of the electromagnetic wave in free space; is the wavelength of the electromagnetic wave in the waveguide; p is the angle measured at the center of curvature from the microwave energy coupling means to the k slot;

R is the radis of arc, and

n is an odd integer.

6. A slotted waveguide antenna as in claim 4 wherein said plurality of radiating slots are spaced longitudinally along said one wall of said curved section of waveguide a distance from said coupling means determined by the equation:

where;

0 is the angle between the plane of radiation and a plane tangent to the curved section of waveguide; 7\ is the wavelength of the electromagnetic wave in free space; R is the wavelength of the electromagnetic wave in the waveguide; qb is the angle measured at the center of curvature from the microwave energy coupling means to the k slot;

R is the radius of curvature; and

n is an odd integer.

7. A slotted waveguide antenna as in claim 2 wherein: said means to couple microwave energy is located a distance from an end of said are; said radiating slots are disposed on both sides of said coupling means; and means are provided to energize said radiating slots on one side of said coupling means in phase opposition to corresponding slots on the other side of said coupling means.

8. A slotted waveguide antenna as in claim 7, wherein said means to energize said radiating slots on one side of said coupling in phase opposition to the corresponding slots on the other side of coupling means comprises a ferrite phase shifter.

9. A slotted waveguide antenna as in claim 8, wherein said ferrite phase shifter is within said curved waveguide section adjacent to said coupling means.

References Cited by the Examiner UNITED STATES PATENTS 2,787,765 4/57 Fox 33324.1 2,807,800 9/57 Broussaud 343771 2,870,418 1/59 Hewitt 333-1.1 2,874,382 2/59 Stavis 343771 2,894,261 7/59 Yarn 34377l HERMAN KARL SAALBACH, Primary Examiner. 

1. A SLOTTED WAVEGUIDE ANTENNA COMPRISING: A CURVED SECTION OF WAVEGUIDE, SAID CURVED SECTION OF WAVEGUIDE HAVING FOUR WALLS, A LONGITUDINAL CENTER LINE, AND A CENTER OF CURVATURE; A PLURALITY OF RADIATING SLOTS SPACED LONGITUDINALLY ALONG ONE WALL OF SAID CURVED SECTION OF WAVEGUIDE, SAID ONE WALL BEING THE WALL FACING AWAY FROM THE CENTER OF CURVATURE; MEANS TO COUPLE MICROWAVE ENERGY TO BE RADIATED INTO SAID CURVED SECTION OF WAVEGUIDE; THE SPACING BETWEN SAID SLOTS VARYING, THE VARIATION IN SPACING BEING A FUNCTION OF THE DISTANCE FROM SAID COUPLING MEANS, WHEREBY SAID ANTENNA RADIATES SAID ENERGY SUB- 